Ordinary water has many advantages for use as a heat-transfer medium in home heating systems, especially those relying at least in part on solar heat energy input. Nevertheless, water is not space-efficient as a thermal energy storage medium; and as a consequence, efforts have been made in the past to provide a compact thermal "battery" or "flywheel" for use in solar hot water heating systems by incorporating therein a vessel filled with an inorganic, liquid-solid phase-change material such as Glauber's salt (Na.sub.2 SO.sub.4 .multidot.10H.sub.2 O) for purposes of storing and selectively releasing heat energy. However, a serious problem exists with these prior art arrangements in that the selected phase-change salt tends to stratify as separate layers of hydrated crystals and anhydrous salt during operation of the system unless mechanical agitation is somehow applied to the salt or to its container. The auxiliary equipment needed for this agitation defeats the objective of space-efficiency and requires a supplemental energy input, usually high-cost electricity.
As an alternative to mechanical agitation, liquid-solid phase-change materials have been proposed which incorporate means for nucleating the thermal energy storage salt. The potentially most useful of these latter systems have proved to be sensitive to the damaging effects of moisture invasion, thus ruling out their utilization heretofore in conventional hot-water heating systems.